1. Field of the Invention
This invention is directed to neutral and low overbased alkylphenoxy sulfonate additive compositions which, at equivalent amounts of diluent oil, have lower viscosities as compared to comparable products known in the art.
2. State of the Art
During operation, the lubricating oil employed in automobile engines accumulates sludge and other harmful deposit forming materials which, if left untreated, would greatly reduce the operating life of the engine. Typically, however, dispersant and detergent additives are added to the lubricating oil to disperse the deposit forming material so as to retard or remove deposit formations. Such additives include, by way of example, alkenyl succinimides, overbased phenates, including overbased sulfurized phenates, neutral and overbased sulfonates, including neutral and low overbased alkylphenoxy sulfonates, and the like. These additives are typically employed in a variety of combinations so that the lubricating oil formulation contains more than one dispersant or detergent to control and/or remove deposit formation.
Of particular interest are neutral and low overbased alkylphenoxy sulfonate additive compositions which are useful in providing detergency and dispersancy properties to lubricating oil compositions. Specifically, it is known that neutral and low overbased alkylphenoxy sulfonates provide for improved control of piston deposits during diesel engine operation as compared to highly overbased alkylphenoxy sulfonates. In addition, low overbased alkylphenoxy sulfonates provide a measure of alkalinity reserve, albeit small, which is useful in neutralizing acids generated during engine operation especially when engines are operating on sulfur containing fuels.
The formation of neutral and low overbased alkylphenoxy sulfonate additive compositions by alkylation of phenol, sulfonation of the alkylated phenolic compounds and subsequent neutralization of the alkylphenol sulfonic acid by at least a stoichiometric equivalent of an alkaline earth metal oxide are, in a very general sense, known in the art.
For example, British Patent Specification No. 1 332 473 discloses the preparation of neutral alkylphenoxy sulfonate additive compositions and further discloses conversion of these materials to overbased alkylphenoxy sulfonate additive compositions (i.e., having a TBN of about 200 or more).
Similarly, U.S. Pat. No. 4,751,010 discloses the preparation of neutral and overbased alkylphenoxy sulfonate additive compositions useful as detergent-dispersant additives in lubricating oils. The disclosed preparation involves the neutralization of an alkylphenoxy sulfonic acid, followed by sulfurizing/overalkalinizing the salt obtained and then carbonating the resultant salt.
Similarly, French Patent No. 2,584,414 relates generally to detergent-dispersant lubricant additives prepared from alkylphenol sulfonic acid by neutralization, sulfurization, overbasing, and carbonation.
While neutral and low overbased alkylphenoxy sulfonate additive compositions are well known in the art, the preparation of these compositions has been substantially hindered by the fact that certain alkylphenoxy sulfonic acids, used as intermediates in the preparation of neutral and low overbased alkylphenoxy sulfonate additive compositions, are unstable at high temperatures (e.g., &gt;50.degree. C.) and/or during prolonged storage/shipment and this instability can result in spontaneous desulfonation. This problem is compounded by the fact that the preparation of alkylphenoxy sulfonic acids by sulfonation of the alkylphenol is generally conducted at elevated temperatures (e.g., 50.degree. C.) and by the fact that it is common to store and/or ship these alkylphenoxy sulfonic acids at ambient conditions over long periods of time. In either case, a significant amount of the alkylphenoxy sulfonic acid can spontaneously desulfonate under these conditions.
Additionally, the commercial utility of neutral and low overbased alkylphenoxy sulfonate additive compositions is hindered by the fact that when prepared by current methodologies, these compositions can possess unacceptably high viscosities which require the further addition of large amounts of diluent to reduce the viscosity prior to the use of these compositions in formulating a complete lubricant package.
Specifically, neutral and low overbased alkyl-phenoxy sulfonate additive compositions have been typically prepared by first preparing the alkylphenol which is conventionally prepared by combining an excess amount of phenol with an olefin or alcohol in the presence of an acidic alkylation catalyst typically having a Hammett (H.sub.o) acidity function of about -2.2 or greater (more positive) and an acid number of about 4.7 milliequivalents or less. Such acidic alkylation catalysts include cross-linked polystyrene sulfonic acid resins (e.g., Amberlyst.TM. 15 resin, available from Rohm & Haas, Inc., Philadelphia, Penna. and which has an H.sub.o value of -2.2 and an acid number of about 4.7 milliequivalents per gram). The resulting alkylphenol is then sulfonated by conventional methodology to form the alkylphenoxy sulfonic acid which, in turn, is reacted with either a stoichiometric or excess amount of an alkaline earth metal oxide in the presence of minimal diluent oil. After completion of the reaction, additional diluent (e.g; diluent oil) is generally added. In any event, because of transport cost considerations, the resulting product preferably should contain no more than about 40 weight percent diluent oil. Under these conditions, however, the viscosity of the neutral alkylphenoxy sulfonate additive composition prepared by prior art techniques is significantly greater than about 1000 cSt at 100.degree. C. and the viscosity of the low overbased salts, while somewhat less than that of the neutral salt, is nevertheless unacceptable.
Consequently, with prior art neutral and low overbased alkylphenoxy sulfonate additive compositions, it is conventional to add further amounts of an appropriate diluent to the additive composition to reduce its viscosity to acceptable ranges or to employ a minor amount of the neutral and low overbased alkylphenoxy sulfonate in combination with a salicylate (see, for example, British Pat. Appl. No. 1 372 532). One diluent typically employed is heavily branched alkylate bottoms (BAB-bottoms) which, by virtue of its branching and relatively low viscosity, lowers the viscosity of the additive composition.
The further addition of a suitable diluent, such as BAB-bottoms, is undesirable because it requires an additional step in the process and increases the cost of the overall process by requiring a component whose primary function is to reduce the viscosity of the additive composition. Likewise, the inclusion of a salicylate additive with a minor amount of a neutral or low overbased alkylphenoxy sulfonate is undesirable because it limits the formulator to using a salicylate in the lubricant composition particularly when the presence of salicylate is either unnecessary or undesirable.
In view of the above, neutral or low overbased alkylphenoxy sulfonate additive compositions having acceptable viscosities with minimal amounts of diluent or no salicylate would provide a significant advantage in the efficient use of these additive compositions. Additionally, neutral and low overbased alkylphenoxy sulfonate additive compositions prepared from alkylphenoxy sulfonic acids having improved stability against desulfonation would provide further advantages in the efficient manufacture, storage and shipment of these additive compositions.